Engine cooling fans develop static pressure across the fan such that regions ahead of the fan are at significantly lower pressure than regions behind the fan. Practical operations of fans used in under-hood engine cooling functions dictates minimum clearances between rotating and stationary components to ensure safe, durable functioning throughout the life of the vehicle. The pressure rise developed across the fan drives leakage flow through the gaps occurring between the fan's blade tips or rotating ring, if present, and the stationary surfaces of the shroud.
In open-blade fans, this leakage flow encounters the tip gap along the entire tip region of each blade from leading edge to trailing edge and enters the gap region having a very high tangential velocity component. As the leakage flow progresses through the gap region, the vicious drag of the fan blade tips continues to strengthen this vertical flow until finally it reaches the exit of the gap region now being radially outward from the blades' leading edge tips. This strong vortex continues to propagate forward, and if not constrained will continue flowing upstream of the fan tangentially and radially outward into the shroud region (adjacent a radiator upstream of the fan assembly) until the primary flow movement recaptures it and pulls it back into the fan passage.
When the recirculation flow reenters the fan passage, it possesses a very high tangential component, which is at great odds with the velocity and direction of the primary incoming flow entering the fan passage through the fan's inlet nozzle. As the tangentially-oriented recirculation flow mixes with the mostly axial primary flow, a vortex is formed just in front of the blade's leading edge at the tip.
Since the leading edge was designed for the primary flow velocity condition, the vortex encountered by the blade is misaligned relative to the intended inlet vector. The above noted action causes the tip region to stall and resulting low relative-momentum flow tends to “hang up” in the blade tip region reducing flow-rate and static pressure and increasing drag and thereby causing efficiency losses.
It is desirable to provide a fan assembly wherein the losses from recirculating leakage flow can be reduced.